BI-EXPONENTIAL NUCLEAR-SPIN-LATTICE RELAXATION IN SOLID HEXAFLUOROACETYLACETONE

The reorientation of CF3 rotors in solid hexafluoroacetylacetone has been studied using pulsed NMR techniques. The temperature dependence of both fluorine (F-19) and hydrogen (H-1) nuclear spin-lattice relaxation has been measured using saturation-recovery pulse sequences. Following saturation of one of the spin reservoirs, the evolution with time of the longitudinal magnetisations associated with both nuclei are observed to be strongly bi-exponential. A theory is presented for the spin-lattice relaxation which is mediated by the heteronuclear dipole-dipole interaction between F-19 and H-1 nuclei. The experimental results are in good quantitative agreement with this relaxation theory. The spectral density functions have been evaluated using the known molecular structure and they are consistent with the experimental data. From this analysis we conclude that the relaxation is dominated by an intramolecular F-19-H-1 dipolar interaction. The spectral density function J(0)(omega(H)-omega(F)) makes an important contribution to the relaxation theory with the result that the motional spectrum is conveniently sampled at the difference frequency between H-1 and F-19 Larmor precessions. Two physically distinguishable CF3 rotors have been identified in this investigation, their dynamics are characterised by activation energies of 7.5+/-0.4 and 13.3+/-0.8 kJ mol-1, respectively.